Geiger Mode Avalanche Photodiodes (GMAPs) have been developed during recent years as a possible alternative to vacuum photomultiplier tubes (PMT) and other solid-state photodetectors. They have many advantages in terms of excellent single photon response, high gain at low bias voltage, high detection efficiency in the visible range, very fast timing response, good temperature and voltage stability, compactness and insensitivity to magnetic fields. Thanks to their properties, GMAPs have found widespread use for the detection of very weak and fast optical signals in many applications like astronomy, fluorescence and luminescence decay measurements, single-molecule detection, and laser ranging.
GMAPs are semiconductor junction diodes reverse-biased a few volts above the breakdown voltage. At this operating condition, the electric field within a GMAP depletion layer is so high that a single carrier injected in this region triggers a self-sustaining avalanche multiplication process. As a result, a sharp current pulse of a few mA and with a sub-nanosecond rise time is produced, thus marking a photon arrival. Once the breakdown current has been detected, it is quenched by a large series resistor (passive quenching) or by a suitable quenching circuit (active quenching). The photodiode is thus turned off for a suitable hold-off time that allows the charge stored within the depletion layer to dissipate. After this time, the reverse bias voltage is restored and the device is ready to detect another photon. Exemplary architectures of GMAP devices are disclosed in the Italian Patent application No. VA2010A000069 in the name of the same applicant, the whole disclosure of which is herein incorporated by reference.
A proximity or distance sensor is a sensor able to detect the presence of nearby objects without any physical contact. It usually includes an infrared pulsed light emitter (typically one or more LEDs) used to illuminate the object.
If no object is close to the sensor, the light is never reflected and the reading shows no object. If the light reflects off an object, it returns to the detector and determines a triangle. The angle identified by the emitter, the point of reflection and the infrared light detector depends upon the distance between the sensor and the object, as schematically illustrated in FIG. 1.
High precision proximity sensors exploiting the very fast timing response of GMAP photodiodes (timing jitter<100 ps) are based on the measurement of the timing distribution of the delays between the true arrival time of photons onto the GMAP's active area and the detection time given by the rising edge of the GMAP's output current pulse. These systems usually employ a Time Correlated Single Photon Counting Apparatus (TCSPC) for the high precision measurements with the start signal given by the electrical trigger of the pulsed light source used to illuminate the object and the stop signal given by the GMAP's output signal. The measurement of the average delay between these times allows a precise and accurate measurement of the distance between the light source and the object.
These high precision distance sensors are typically used for night vision mainly in military applications, exploiting the single photon sensitivity in the dark environments of GMAP detectors. They turned out to be extremely complicated, expensive and not fully suitable for commercial applications.